linux-kernel - RE: [RFC][PATCH v2 2/2] driver core: Ensure wait_for_device_probe() waits until the deferred_probe

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Date:   Wed, 8 Apr 2020 10:29:20 +0000
From:   Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:     John Stultz <john.stultz@...aro.org>,
        lkml <linux-kernel@...r.kernel.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Rob Herring <robh@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        netdev <netdev@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: RE: [RFC][PATCH v2 2/2] driver core: Ensure wait_for_device_probe()
 waits until the deferred_probe_timeout fires

Hi John,

> From: John Stultz, Sent: Wednesday, April 8, 2020 4:27 PM
> 
> In commit c8c43cee29f6 ("driver core: Fix
> driver_deferred_probe_check_state() logic"), we set the default
> driver_deferred_probe_timeout value to 30 seconds to allow for
> drivers that are missing dependencies to have some time so that
> the dependency may be loaded from userland after initcalls_done
> is set.
> 
> However, Yoshihiro Shimoda reported that on his device that
> expects to have unmet dependencies (due to "optional links" in
> its devicetree), was failing to mount the NFS root.
> 
> In digging further, it seemed the problem was that while the
> device properly probes after waiting 30 seconds for any missing
> modules to load, the ip_auto_config() had already failed,
> resulting in NFS to fail. This was due to ip_auto_config()
> calling wait_for_device_probe() which doesn't wait for the
> driver_deferred_probe_timeout to fire.
> 
> This patch tries to fix the issue by creating a waitqueue
> for the driver_deferred_probe_timeout, and calling wait_event()
> to make sure driver_deferred_probe_timeout is zero in
> wait_for_device_probe() to make sure all the probing is
> finished.
> 
> The downside to this solution is that kernel functionality that
> uses wait_for_device_probe(), will block until the
> driver_deferred_probe_timeout fires, regardless of if there is
> any missing dependencies.
> 
> However, the previous patch reverts the default timeout value to
> zero, so this side-effect will only affect users who specify a
> driver_deferred_probe_timeout= value as a boot argument, where
> the additional delay would be beneficial to allow modules to
> load later during boot.
> 
> Thanks to Geert for chasing down that ip_auto_config was why NFS
> was failing in this case!
> 
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Alexey Kuznetsov <kuznet@....inr.ac.ru>
> Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Rafael J. Wysocki <rjw@...ysocki.net>
> Cc: Rob Herring <robh@...nel.org>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
> Cc: netdev <netdev@...r.kernel.org>
> Cc: linux-pm@...r.kernel.org
> Reported-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
> Fixes: c8c43cee29f6 ("driver core: Fix driver_deferred_probe_check_state() logic")
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
> * v2: Split patch, and apply it as a follow-on to setting
>       the driver_deferred_probe_timeout defalt back to zero
> ---

Thank you for the patch! This patch doesn't cause any side
effect on my environment (the value of driver_deferred_probe_timeout is 0).
So,

Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>

Best regards,
Yoshihiro Shimoda